(1) Field of the Invention
The present invention relates to an apparatus and method which controls a wavelength-multiplex optical amplifier by executing an automatic level control, so that the output optical power is maintained at a constant level.
(2) Description of the Related Art
In a wavelength-multiplex optical amplifier control apparatus which utilizes wavelength-division multiplexing (WDM), an automatic level control (ALC) is executed by varying a gain of an optical amplifier so as to maintain the output optical power at a constant level. The execution of the ALC on the optical amplifier, which maintains the output optical power at the constant level even when the input optical power changes, makes it possible to avoid the occurrence of an error of the multiplexed data at a downstream node due to fluctuation of the optical signal power. In addition, if the ALC is performed, the output optical power can be maintained at the constant level even when the input optical power considerably changes due to different transmission losses at different transmission distances of trunk nodes in an optical fiber transmission system.
Generally, when the ALC is executed during the wavelength-multiplex amplifier control, it is necessary that the output optical power of each of the individual channels, contained in the optical signal output by the optical amplifier, be monitored in order to appropriately adjust the power gain and maintain the output optical power at the constant level. However, as the number of channels in the multiplexed data increases, the hardware having an increasingly large size is needed to monitor the output optical power for each individual channel, and it will become expensive.
FIG. 1 shows a conventional wavelength-multiplex optical amplifier control apparatus.
In the conventional wavelength-multiplex optical amplifier control apparatus of FIG. 1, an input optical signal carrying the wavelength-division multiplexed data is supplied to an optical amplifier 10. An automatic level control (ALC) circuit 12 outputs a control signal to the optical amplifier 10 by executing the ALC. The optical amplifier 10 amplifies the input optical signal by varying the gain according to the received control signal, and outputs the amplified optical signal to an optical splitter 14.
Alternatively, the optical amplifier 10 may include a variable attenuator, and the ALC circuit 12 may output a control signal to the variable attenuator. The amount of attenuation of the input optical signal at the variable attenuator is varied according to the control signal output by the ALC circuit 12. The optical amplifier 10 amplifies the input optical signal by a given gain, and the amplified optical signal is processed through the variable attenuator. The optical amplifier 10 supplies the output of the variable attenuator to the optical splitter 14 so that the output optical power is maintained at the constant level.
In the conventional wavelength-multiplex optical amplifier control apparatus of FIG. 1, the ALC circuit 12 outputs a control signal to the optical amplifier 10 so that the output optical power is maintained at a constant level according to the control signal. However, if an optical power of a certain channel in the multiplexed data is partly turned off or partly changes at an upstream node, the output optical power at the output of the optical amplifier 10 is maintained at the same constant level without taking account of the off-state or change of the channel. An error of the remaining channels in the multiplexed data will occur at a downstream node due to the fluctuation of the optical signal power.
In order to avoid the occurrence of such an error, it is necessary to transmit a monitoring control signal, indicating the off-state or change of one channel in the multiplexed data, to the optical amplifier 10 of the control apparatus of FIG. 1 as well as to the optical amplifiers of the downstream nodes, in addition to the transmission of the multiplexed data stream. By receiving the monitoring control signal at the conventional wavelength-multiplex optical amplifier control apparatus of FIG. 1, the ALC circuit 12 outputs a corrected control signal to the optical amplifier 10 according to the monitoring control signal, such that the output optical power is maintained at a suitably corrected level according to the control signal. This makes it possible to avoid the occurrence of an error of the remaining channels at the downstream node.
However, in a case in which the transmission of the monitoring control signal is used, if the monitoring control signal is erroneously not transmitted (or erroneously set in an off-state) and an optical power of a primary channel in the multiplexed data is turned off or changes, an error of the remaining channels in the multiplexed data may occur at a downstream node, because the ALC is executed at the control apparatus of FIG. 1 without taking account of the off-state or change of the primary channel.
Further, in the case in which the transmission of the monitoring control signal is used, the downstream nodes have the delay in receiving the monitoring control signal while the monitoring control signal is terminated at each node. It is necessary that the execution of the ALC by the control apparatus of FIG. 1 is started at an adequately delayed timing. If the number of remote nodes in the optical transmission system increases, the delay in receiving the monitoring control signal at the remote nodes becomes increasingly large. Hence, the start of the execution of the ALC at the final node may be considerably delayed until the monitoring control signal is received by the final node.
Accordingly, in a case in which a speedy execution of the ALC at the downstream nodes and a fail-safe function of the optical transmission system are desired, the transmission of the monitoring control signal is not used, and a spectrum analyzer is needed for the conventional wavelength-multiplex optical amplifier control apparatus at each node. The spectrum analyzer acts to monitor the output optical power of each of the individual channels, contained in the output signal of the optical amplifier, in order to adjust the power gain and maintain the output optical power at the constant level. However, the implementation of a spectrum analyzer into each of the nodes of the optical transmission system will be considerably expensive.